1. Field of the Invention
The present invention relates to a secondary electron multiplier for detecting ions and electrons in a space by detecting secondary electrons drawn out from a secondary electron multiplying tube made of a semiconductive material having a secondary electron emissive property.
2. Description of the Related Art
Recently, so called channel electron multipliers are widely used for detecting ions and electrons in the cosmic space or detecting various ions and atoms in the mass spectrometers.
As shown in FIG. 6, the channel electron multiplier of this type provides a secondary electron multiplying tube 2 made of a semiconductive material having a secondary electron emissive property. The secondary electron multiplying tube 2 provides a funnel portion 2a for entering charged particles P such as ions; electrons on the outer surface of which an input electrode 1 is formed. A collector 3 formed by a circular metal plate is arranged so as to opposed an output end 4 of the tube 2 with a gap g of 0.5 to 1.0 mm. In operation thereof, a high negative voltage E.sub.H of (-3) to (-4) kV is applied to the input electrodes 1 and a low negative voltage E.sub.L (-100) to (-200) V is applied to an exit or output electrode 4 formed on the output end of the tube 1 in order to collect a multiplied electron current by the collector 3. This collector voltage E.sub.L is obtained by a voltage drop caused by a resistance R which is connected between the exit electrode 4 and the earth. The current of secondary electrons collected by the collector 3 is charged into a stray capacitance Cs generated between the collector 3 and the earth and, then, the charged electron in the stray capacitance Cs is discharged through a load resistance R.sub.L. A voltage generated between both ends of the load resistance R.sub.L at that time is amplified by a first amplifier 5.
FIG. 7 shows a concrete structure of a conventional secondary electron multiplier. Two elongated insulating plates 6 and 7 are fixed in parallel with each other using three studs 8, 9 and 10 and vises 11 and the secondary electron multiplying tube 2 is supported between them. More concretely, it is supported at the neck position of the funnel portion 2a and a position near the output end 4 thereof by support metal bands 12 and 13 which are welded on the,studs 8 and 10, respectively. The input electrode 1 and the exit electrode 4 of the tube 2 are electrically conducted to the studs 8 and 10 through the support metal bands 12 and 13, respectively. The collector 3 is welded onto the stud 9 so as to face the output end of the tube 2. A terminal metal plate member 14 is fixed between respective ends of the insulating plates 6 and 7 using vises 11 and nuts 15 and, between other ends of the insulating plates 6 and 7, a face plate 16 having an aperature 16a for conducting ions and a Faraday cup Fc is fixed using vises 11 and nuts 15. The Faraday cup Fc is provided for shielding the multiplier from the soft X-ray photons and UV-photons generated in the ion source. A positive voltage E'.sub.H is applied to the face plate 16 as shown in FIG. 6. Also, the Faraday cup Fc repells the ion current coming from the faceplate toward the funnel of the secondary electron multiplier. The terminal metal plate member 14 has first to third hermetical terminals 17, 18 and 19. The first terminal 17 is electrically conducted to the face plate 16 by a lead wire 21 and the second and third terminals 18 and 19 are electrically conducted to studs 8 and 9 by lead wires 22 and 23, respectively. Further, the resistance R is electrically connected between the terminal metal plate member 14 and the stud 10 by lead wires 24a and 24b.
Meanwhile, in the conventional secondary electron multiplier shown in FIG. 7, the neck portion of the funnel portion 2a of the tube 2 is fixed to the stud 8 by the metal band 12 which is welded thereto. Namely, the metal band 12 is wound around the neck portion of the funnel portion 2a and is fastened thereto using a vis 24 and a nut 25 and, thereafter, the position of the funnel portion 2a of the tube 2 toward the Faraday cup Fc are determined according to a predetermined standard so as to face the Faraday cup Fc. Then, the free end of the metal band 12 is fixed to the stud 8 by the spot welding.
However, in the structure of the conventional secondary electron multiplier mentioned above, it is difficult to weld the metal band 12 to the stud 8 maintaining the position thereof determined according to the predetermined standard and, therefore, it requires much skill.
Further, in the conventional secondary electron multiplier mentioned above, the negative high voltage E.sub.H is applied to the input electrode 1 through the second terminal, the lead wire 22, the stud 8 and the metal band 12. Since it is necessary to apply the negative high voltage E.sub.H to the end surface of the mouth of the funnel portion 2a, it becomes necessary to form an insulating membrane (not shown) on the outer surface of the funnel portion 2a in which the input electrode 1 is formed. Due to this, the cost of the product comes high.